1. Field of the Invention
Aspects of the invention relate to an image forming apparatus with a laser scanning device and a method of aligning the laser scanning device, and more particularly to an image forming apparatus equipped with a laser scanning device capable of performing bidirectional laser scanning using a resonant scanning mirror, and a method of aligning the laser scanning device.
2. Description of the Related Art
In general, an image forming apparatus is a device that prints out black-and-white images or color images onto a recording medium such as paper in accordance with image signals. Examples of such an image forming apparatus include a laser printer, an inkjet printer, a photocopier, a digital multifunction printer, a facsimile machine, and the like. The various image forming apparatuses create an image generally by an electrophotographic method or an inkjet method. In the electrophotographic method, an electrostatic latent image is formed on a photoconductive medium by scanning the photoconductive medium with a laser beam. Then, developer is adhered to the photoconductive medium corresponding to the electrostatic latent image, and a thus-formed developer image is transferred to a recording medium, thereby forming a final image. The inkjet method forms an image by spraying liquid ink on a recording medium in accordance with image signals.
An electrophotographic image forming apparatus forms a desired image using the following process. First, a surface of the photoconductive medium is charged to a predetermined electric potential. A laser beam is emitted onto the surface of the charged photoconductive medium to thereby form an electrostatic latent image. A visible image is formed by adhering powder-type developer to the electrostatic latent image. Next, the visible image formed by the developer is transferred to a recording medium and fixed on the recording medium by heat and pressure.
The electrophotographic image forming apparatus is equipped with a laser scanning device that forms the electrostatic latent image by scanning the photoconductive medium with a laser beam in accordance with image signals. The laser scanning device includes a light source that generates the laser beam according to the image signals, a collimator lens that forms a parallel laser beam from the laser beam emitted from the light source with respect to an optical axis, a cylinder lens that converts the parallel laser beam emerging from the collimator lens to a linear laser beam that is horizontal with respect to a vertical scanning direction, a polygon mirror that reflects the linear laser beam emerging from the cylinder lens and scans the reflected laser beam in a horizontal scanning direction, an fθ lens that focuses the reflected laser beam scanned by the polygon mirror on the photoconductive medium by compensating for an aberration of the scanned laser beam, and a detector that detects the scanned laser beam and generates a synchronizing signal each time the scanned laser beam is detected. These component parts are all mounted in a single frame as one integrated unit.
Recently, the tendency has been to increase the rotation speed of the polygon mirror to improve a printing speed of the image forming apparatus. Therefore, the cost to achieve this has been increasing.
To this end, there has been introduced a method of employing a flat mirror having a reflection surface, which is relatively inexpensive, instead of the polygon mirror rotating at a high speed, as disclosed in U.S. Patent Application Publication No. 2005-0275710 published on Dec. 15, 2005.
According to the disclosed method, a photoconductive medium is scanned in accordance with image signals in a forward direction and then a backward direction using a resonant scanning mirror in which the reflection surface that reflects the laser beam is oscillated at a resonant frequency.
In order to accurately form the electrostatic latent image on the photoconductive medium according to the image signals, all component parts of a laser scanning device including the resonant scanning mirror, including an incident optical system including the collimator lens and the cylinder lens, must be precisely and stably mounted on a housing.
When the resonant scanning mirror is used to deflect the laser beam, synchronizing signals are generated using different detectors corresponding to the different scanning directions, that is, the forward scanning direction and the backward scanning direction. Thus, the positions of the detectors mounted on the housing as well as the position of the resonant scanning mirror oscillating at the resonant frequency to deflect the laser beam must be strictly controlled.
For accurate generation of the synchronizing signals, the plurality of detectors generating the synchronizing signals and the resonant scanning mirror must be placed at correct positions. If an error occurs in mounting the resonant scanning mirror on the housing during assembly, the resonant scanning mirror may not be synchronized properly, or the scanning may be performed according to incorrectly generated synchronizing signals. Accordingly, the ability to check the position of the resonant scanning mirror after assembly is required.